The present invention relates to a method for preparing fatty acid silver salt and a heat-developable photosensitive material incorporating the fatty acid silver salt obtained by such preparation method.
A strong need for reducing the volume of waste process solution has arisen in recent medical field from viewpoints of environmental preservation and space saving. Thus a technology related to a photosensitive heat-developable photographic material for medical diagnosis and photographic purposes has been desired, the material being such that affording efficient light exposure with a laser image setter or laser imager, and providing a black image with high resolution and sharpness. Such photosensitive heat-developable photographic material can provide the user with a more simple and environment-conscious image producing system using no solution-base process chemicals.
The image producing method based on heat development is disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and xe2x80x9cThermally Processed Silver Systemsxe2x80x9d written by D. Morgan and B. Shely, Imaging Processes and Materials, Neblette""s 8th ed., edited by Sturge, V. Walworth and A. Shepp, p.2, (1989).
Such photosensitive material contains an organic reducible non-photosensitive silver source (e.g., organic acid silver salt), a catalytic amount of photocatalyst (e.g., silver halide) and a reducing agent for silver, all of which being generally dispersed in an organic binder matrix. While the photosensitive material is stable at the room temperature, it will produce silver through a redox reaction between the reducible silver source and the reducing agent when heated to a high temperature (80xc2x0 C. or above, for example) after light exposure. The redox reaction is promoted by a catalytic action of the latent image produced by the light exposure. That is, the silver generated by the reaction of the reducible silver within the exposed area provides a black spot, which makes a contrast with the non-exposed area and is recognizable as an image.
The silver source employed by such system generally refers to a silver salt of a fatty acid, and a variety of methods for producing thereof have been known. Examples of the methods include such that preparing an organic acid silver salt in a concomitant solution of water and water-insoluble solvent as disclosed for example in JP-A-49-93310 (the code xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-49-94619 and JP-A-53-68702; such that preparing an organic acid silver salt in an aqueous solution as disclosed in JP-A-53-31611, JP-A-54-4117 and JP-A-54-46709; such that preparing an organic acid silver salt in an organic solvent as disclosed in JP-A-57-186745, JP-A-47-9432 and U.S. Pat. No. 3,700,458. In principle, the organic acid silver salt is obtained by dissolving a fatty acid into water under heating to a temperature of the melting point thereof or above, adding sodium hydroxide or an alkali metal salt under vigorous stirring, and further adding silver nitrate to convert an alkali soap into a silver soap.
The alkali soap forms micell in the aqueous solution, which appears as a milky liquid. The conversion reaction from such micellar state to silver salt, however, often suffers from a problem in production stability. Thus as a measure for obtaining a homogeneous solution of alkali soap, use of a mixed solution of water and alcohol as a solvent is disclosed in JP-A-55-40607.
Now the alkali soap shows alkalinity as its name suggests, and is prepared under a high pH environment. Adding silver nitrate to an alkali solution, however, not only produces silver oxide as a by-product but also results in an undesirable production of silver nucleus by an action of a trace amount of contaminant which inevitably generates during the production and exhibits a high reducing activity under such high-pH environment. Such by-product is quite disadvantageous in that degrading property of the heat-developable photographic material and in particular in that causing undesirable fog. From this viewpoint, a method for obtaining a homogeneous solution to suppress the generation of the by-product is disclosed in JP-A-55-40607, in which fog still remains unsolved.
In JP-A-9-127643, disclosed is a method for producing silver salt based on simultaneous measuring and addition of an alkali metal salt solution and silver nitrate solution, and is specified as simultaneous addition of aqueous sodium behenate solution and isopropyl alcohol. While the method is successful in at least lowering the high pH during the reaction to the medium range and thereby in suppressing the generation amount of silver oxide, fog still cannot totally be cleared due to a weak reducibility of isopropyl alcohol.
As described above, preparation of fatty acid silver salt needs special accounts such that eliminating as possible reducible substances during the formation of fatty acid silver salt, controlling the grain size and controlling the grain form, where all of them cannot be satisfied at a time by the conventional method.
In the production of a heat-developable photosensitive material using the fatty acid silver salt, a photosensitive layer thereof is often formed by coating a coating liquid containing an organic solvent such as toluene, methyl ethyl ketone or methanol. Using an organic solvent as the solvent, however, is not only disadvantageous in terms of safety in the production processes, adverse effects on human body, and high cost ascribable to the solvent recovery or the like, but is also inappropriate in terms of providing an environment-conscious heat-developable photosensitive material.
Thus a method for forming the photosensitive layer using a water-base coating liquid. For example, JP-A-49-52626 and JP-A-53-116144 disclose cases using gelatin as a binder. In JP-A-50-151138, a case using polyvinyl alcohol as a binder is described.
A case with a combined use of gelatin and polyvinyl alcohol is found in JP-A-60-61747. As another exemplary case, the photosensitive layer using a water-soluble polyvinyl acetal as a binder is described in JP-A-58-28737.
As is clear from the above, using a water-soluble binder allows the photgosensitive layer to be formed with awater-base coating liquid and is beneficial from environmental and economic viewpoints. The water-soluble polymer binder is, however, less compatible with the fatty acid silver salts, which will fail in obtaining a coated film with a surface quality agreeable to the practical use, will result in brownish to yellowish tone of the silver image after the development afar from intrinsically preferable black tone and will result in increased fog. Thus only afforded was a heat-developable photosensitive material whose property being significantly degraded and commercially unsuccessful.
In order to obtain practically agreeable quality of the coated surface using the water-base coating liquid containing a fatty acid silver salt, the fatty acid silver salt must be kept in a finely dispersed state in the water-base solution without agglomeration. Discovery of a method for finely dispersing the fatty acid silver salt is thus desired. One method generally accepted relates to such that producing a hydrophobic grain dispersion of a fatty acid silver salt, separating the grain therefrom by filtration to obtain a solid matter, and re-dispersing the solid matter after being mixed with a dispersing agent as described by D. Kloosterboer in Imaging Processes and Materials, Neblette""s 8th ed., edited by Sturge, V. Walworth and A. Shepp, p.279, (1989).
Fine dispersion operation of the fatty acid silver salt can be effected by mechanical dispersion in the presence of a dispersing agent using a known pulverizing means (e.g., high-speed mixer, homogenizer, high-speed impact mill, banbury mixer, homomixer, kneader, ball mill, vibration ball mill, epicyclic ball mill, attritor, sand mill, bead mill, colloid mill, jet mill, roller mill, trommel and high-speed stone mill). These methods, however, produce only a coating liquid including a lot of agglomerated grains and thus causative of degraded surface quality, and, worse than all, tend to indiscriminately cleave the primary grains of the fatty acid silver salt which is originally crystallized as a water-insoluble salt, so that excessive silver nuclei are generated on the crystal cleavage plane of the grains and thereby to increase fog.
On the other hand, JP-B-7-119953 (the code xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese Patent Publicationxe2x80x9d), JP-A-8-137044 and JP-A-8-238848 disclose methods such that finely dispersing the fatty acid silver salt by pressure treatment. The methods, however, relate to an organic solvent-base dispersion and stand on a basis different from solving the foregoing problem.
In JP-A-9-127643, disclosed is a method such that obtaining a dispersion of the fatty acid silver salt by simultaneous measuring and addition of an alkali metal salt solution and silver nitrate solution, and then directly desalting the dispersion by dialysis or ultra-filtration. This method is preferable at least in that the primary grain obtained in the crystallization process of the fatty acid silver salt can be incorporated as intact into the photosensitive layer without being crushed. The method, however, still suffers from problems in agglomeration of the grain under a condition of high salt concentration, and in thickening during concentration of the dispersion, which makes the method difficult to be accepted as a measure for obtaining a practical coating liquid.
As described above, a stable method for preparing a coating liquid containing in a solvent a fatty acid silver salt capable of affording an excellent coated surface quality and optical properties such as low haze and low fog has not been undiscovered.
It is therefore an object of the present invention to provide a method for preparing a fatty acid silver salt excellent in decreased fog property when incorporated into a heat-developable photographic material, a time-dependent decreased fog property, image stability after the heat development, and light transmissivity. It is a further object of the present invention to provide a method for preparing a stable dispersion of the fatty acid silver salt, which is capable of improving the coated surface quality, without separating nor re-dispersing such salt as a solid matter.
The present inventors found after extensive investigations for achieving the above object that, in a method for producing a non-photosensitive fatty acid silver salt grain comprising the steps of reacting a silver ion-containing solution and a solution of alkali metal salt of a fatty acid to prepare a fatty acid silver grain, adding thereto a dispersing agent, and desalting the obtained dispersion of the fatty acid silver salt by ultra-filtration, it is possible to prepare the dispersion of the fatty acid silver salt having an excellent property by controlling the pH and temperature of the dispersion within a predetermined range after the addition of the dispersing agent; and also found that a heat-developable photosensitive material using thus-obtained fatty acid silver salt can exhibit an excellent photographic property, which led us to propose the present invention.
That is, according to a first aspect of the present invention, provided is a method for producing non-photosensitive fatty acid silver salt comprising the steps of reacting a silver ion-containing solution, the solvent of which being a mixture of an organic solvent and water or water, with a solution of an alkali metal salt of a fatty acid, solvent of which being water, organic solvent or a mixture of an organic solvent and water, to obtain a fatty acid silver grain; adding a dispersing agent; and desalting the obtained fatty acid silver dispersion by ultra-filtration; characterized in that pH of the dispersion is kept at 6 or above during a period from a point of time immediately after the addition of the dispersing agent to a point of time an electric conductivity of the filtrate drops below 2000 xcexcS/cm but not lower than 500 xcexcS/cm by the desalting operation.
In the present invention, the pH of the dispersion is preferably kept within a range from 6 to 8 during a period from a point of time immediately after the addition of the dispersing agent to a point of time an electric conductivity of the filtrate drops below 2000 xcexcS/cm but not lower than 500 xcexcS/cm by the desalting operation.
In the present invention, temperature of the dispersion of the fatty acid silver grain is preferably kept within a range from 1 to 250xc2x0 C., and more preferably 5 to 20xc2x0 C., during a period from a point of time immediately after the addition of the dispersing agent to a point of time the desalting operation ends.
In one embodiment of the present invention, the ultra-filtration is continued while adding a poor solvent of the dispersing agent after the electric conductivity of the filtrate drops below 1000 xcexcS/cm but not lower than 500 xcexcS/cm by the desalting operation. The poor solvent of the dispersing agent is preferably methanol or ethanol.
In the present invention, the dispersion is preferably concentrated to a dispersoid content of 10 to 70 wt %, and more preferably 20 to 50 wt %, after the electric conductivity of the filtrate drops below 300 xcexcS/cm but not lower than 20 xcexcS/cm by the desalting operation.
In the present invention, silver preferably exists in excess of alkali metal by 1 to 20 mol % after the reaction.
In the present invention, the dispersing agent is preferably a nonionic amphiphilic substance.
In the present invention, the dispersing agent is preferably added in an amount of 1 to 30 wt %, and more preferably 3 to 20 wt % of the dispersoid.
In the present invention, concentration of the fatty acid silver grain immediately after the reaction is preferably 1 to 10 wt %.
In the present invention, immediately after the desalting operation is completed, the dispersion is concentrated to a fatty acid silver grain content of 15 to 40 wt %, and more preferably 15 to 25 wt %.
In the present invention, sphere-equivalent diameter of the fatty acid silver grain immediately after the reaction is preferably 0.1 to 0.8 xcexcm.
In the present invention, long edge/short edge ratio of the fatty acid silver grain immediately after the reaction is preferably 1 to 4.
In the present invention, aspect ratio of the fatty acid silver grain immediately after the reaction is preferably 2 to 30.
In the present invention, thickness of the fatty acid silver grain immediately after the reaction is preferably 0.01 to 0.20 xcexcm.
According to a second aspect of the present invention, provided is a heat-developable photosensitive material containing a non-photosensitive fatty acid silver salt, a reducing agent for silver ion, a binder and a photosensitive silver halide grain, characterized in that the non-photosensitive fatty acid silver salt is such that prepared by the method of the present invention.